(1) Field Of The Invention
The present invention relates to oriented polyester film coated in-line on at least one side with a water soluble primer coating composition comprising a hydrolyzed aminofunctional silane which renders the film more receptive to extrusion coating with other polymeric film materials such as polyolefins.
(2) Prior Art
Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), has been widely used as a packaging material or as a base for microfilm, reprographic films, proofing films, and the like. It has good optical clarity and toughness which renders it extremely suitable for such applications.
A major use of PET film is in laminations with other polymers and with paper or foil. Often the PET film is extrusion coated with polyethylene or an ethylene copolymer in order to get heat sealing capability, adhesion to another material such as aluminum foil, additional bulk (thickness) or stiffness, or to achieve properties not possible with PET alone.
Unfortunately, the surface of PET film is not very receptive to extrusion coating with other polymers. Numerous primer coatings are known in the prior art for application to the surface of polyester film to improve the adhesion of the film to various materials. Examples of such coatings include compositions based on vinylidene chloride polymers (U.S. Pat. No. 2,698,240), thermosetting acrylic or methacrylic polymers (U.S. Pat. No. 3,819,773) and like materials. Certain water dispersible copolyesters have also been disclosed as adhesives for laminating sheets of polyester film to one another or to sheets of nylon film, as taught in U.S. Pat. Nos. 3,563,942 and 3,779,993. More commonly, solvent based polyurethane adhesives and polyethyleneimine primers are used with attendant problems with solvent emissions and safety. Corona discharge treatment is also used with and without a primer to achieve adhesion between the extrudates.
In common practice, for extrusion coating, a converter will corona treat the PET film as received from the manufacturer, primer coat the corona treated film, dry the primer coat, and then extrusion coat another polymer onto the film (treating film "off" the manufacturing line is commonly referred to as "off-line", treating film during the manufacturing process is commonly referred to as "in-line"). The need to prime the film introduced an additional process step that requires expensive additional equipment and affects yields in off-line treatment accomplished by a converter. In addition, if a solvent based primer is required, additional safety and pollution equipment is required and an additional regulatory burden is imposed.
For the above reasons it would be advantageous for the converter to be able to obtain an already treated or primed film that was directly extrusion coatable without additional priming or corona treatment. To keep from incurring the same costs and problems it is also advantageous that the directly extrusion coatable film be produced by treating or primer coating the film in-line during the manufacturing of the film. For an in-line process it would be ideal for the primer coating to be water based for safety and health reasons, reclaimable (i.e., not cause excessive yellowing or deterioration in physical properties when primer coated film scrap is mixed with fresh polymer and reextruded, which is necessary for an in-line process because of low conversion efficiencies), and hard enough to allow the film to be wound into a roll (as would be necessary for a directly extrusion coatable film) without sticking or blocking.
A directly extrusion coatable polyester film is disclosed in U.S. Pat. No. 4,410,600 to P. T. McGrail. Disclosed is a biaxially oriented PET film coated in-line with a crosslinked styrene-maleic anhydride copolymer. The coated film, however, still requires corona discharge treatment prior to extrusion coating.
The use of silane coupling agents to improve the bonding properties of polyethylene sheets and polyester sheets is known. For example, polyester sheets and polyethylene sheets primed with either vinyltrimethoxysilane or chloropropyltrimethoxysilane have been successfully laminated using a hot melt adhesive such as an ethylene/vinyl acetate terpolymer or an elastomeric polyester as disclosed by E. Plueddemann, "Bonding Through Coupling Agents", Plenum Press, New York, 1985. In addition, the use of N-2-aminoethyl-3-aminopropyltrimethoxysilane (sold under the trademark Z-6020 by Dow Corning) as a primer coating to enhance the adhesion of an ionomer resin (salt of a copolymer of ethylene and methacrylic acid) to glass and to polycarbonate sheets is disclosed in U.S. Pat. No. 4,663,228. Related silanes such as N-3-aminopropyltrialkoxysilanes are known to enhance the adhesion between polyurethane films and glass substrates, as disclosed in published European patent application No. 171,917.
None of these prior art disclosures addresses the problem of direct extrusion coatability of film, particularly of polyester film to polyethylene, ethylene copolymers and ionomers, or other polymer films, without additional primers or additional corona treatment.
Accordingly, it is a primary object of this invention to provide a primer coating which is applied in-line during the polyester film manufacturing process.
Accordingly, it is an object of this invention to provide an oriented, polyester film that can be directly extrusion coatable with other polymers without the need for additional primers or additional corona treatment.
Accordingly, it is also an object of this invention to provide a polyester film having a primer coating thereon in which the primer coated film scrap can be reclaimed without causing excessive yellowing or deterioration in properties of the reextruded polymer.
Most prior art primer coatings employ volatile solvents, thus requiring associated equipment to handle the solvents, associated equipment to protect the workers, and associated equipment to dispose of any waste solvent.
Accordingly, it is another object of the present invention to develop an aqueous primer coating that can be incorporated into most existing systems without the need for expensive associated equipment.